In the technology of insulated wires, especially coaxial cables for high-frequency signal transmission, improvements by way of enhancing the expansion of cellular (or foamed) insulation are desirable in order to decrease leakage attenuation and clarify image and sound.
A typical cable is constructed of metal conductors insulated with a polymeric material. These insulated conductors are generally twisted to form a core and are protected by another polymeric sheath or jacket material. In certain cases, added protection is afforded by inserting a wrap between the core and the sheath. In fiber optics cable, glass fibers are used instead of metal conductors, but a protective sheath is still necessary. A typical coaxial cable is comprised of an inner conductor, typically copper or copper clad steel or aluminum; a dielectric insulation layer; and an outer conductor, for example, aluminum foil with aluminum or copper braid or tube.
The general practice for producing cable insulated with expanded cellular (or foamed) coatings is to use an expansion or blowing process, which relies on chemical or gaseous blowing agents.
A chemical blowing process comprises the following steps: a chemical blowing agent is blended with a resin component at a temperature below the decomposition temperature of the blowing agent; the blended material is fed to an extruder for coating onto a conductor at a temperature above the decomposition temperature of the blowing agent; and the coated layer is subsequently allowed to expand in air and solidify using a coolant such as water. Chemical blowing processes are prevalent to a certain extent because the required investment is lower and the operation is simpler than gas blowing processes in spite of the smaller expansion and lower mechanical strength than is the case with gas blowing processes.
The extent of expansion by a chemical blowing process is, at most, 70 percent by volume. In addition, current processes use high pressure low density polyethylene (HP-LDPE). The expanded products of these processes do not have enough mechanical strength and are unsatisfactory for producing coaxial cables of high quality.
On the other hand, gas blowing processes use, in place of a chemical blowing agent, a chlorofluorocarbon gas such as monofluorotrichlormethane, difluorodichloromethane, trifluorotrichloroethane, and tetrafluorodichloroethane to easily obtain highly expanded products of 80 percent (by volume) expansion or more. Since these processes utilize high density polyethylene, enough mechanical strength is provided to produce coaxial cables of high quality. Chlorofluorocarbon gases, however, because of their negative effect on the ozone layer, are being phased out. Inert gases such as nitrogen, argon, and carbon dioxide have been proposed as alternatives to the chlorofluorocarbon gases. Attempts to substitute nitrogen for the chlorofluorocarbons, however, have resulted in expanded products, which do not have a uniform and fine cell structure. These expanded products are not commercially satisfactory.
It is also found that low density homopolymers of ethylene made by a high pressure process provide mechanical strength inferior to high density polyethylene.